The invention relates to a process for the preparation of dialkoxythiophenes and alkylenedioxythiophenes by decarboxylation of dialkoxythiophenedicarboxylic acids and alkylenedioxythiophenedicarboxylic acids, respectively, in solvents that have a higher boiling point than the product and that contain no nitrogen bases.
U.S. Pat. No. 2,453,103 (DuPont, 1948) describes the thermal decarboxylation of 3,4-dimethoxythiophene-2,5-dicarboxylic acid in quinoline at 180-185xc2x0 C. with addition of a special Cu powder. However, the presence of amines in the end product, even in traces, must be avoided since they interfere in the following step. The workup is therefore carried out by washing with water and acid. The basic quinoline enters the waste water as a salt, which causes environmental pollution or makes an additional, complex step necessary to recover the basic quinoline from the aqueous phase. Even replacement of the Cu catalyst by Cu/Cr oxide (see E. Fager, J. Amer. Chem. Soc., 67 (1945), 2217-2218) does not give better results (58% yield after decarboxylation in quinoline at 180xc2x0 C. and aqueous/acidic workup).
The same difficulty applies to the procedure of M. Coffey et al., Synthetic Communications, 26 (11), 2205-2212 (1996), method 2, which, above all, reveals that Cu (as copper bronze) must advantageously be used in suitable amounts, namely in 1 part per 4 parts of dicarboxylic acid. The requisite temperature of 180-200xc2x0 C. requires considerable expenditure of energy and apparatus that are not available everywhere. The yield achieved for 3,4-ethylenedioxythiophene (EDT) is only 54%, which is inadequate for industrial application.
This also applies to an increased extent to the method of U.S. Pat. No. 2,453,103, which requires heating at the melting point of the dicarboxylic acid employed for from 2 to 4 hours. The EDT-dicarboxylic acid melts only at temperatures above 300xc2x0 C. Considerable formation of tar-like materials occurs, which makes the purification by crystallization described in method 1 very difficult and high in losses. In fact, this method is not even described for 3,4-EDT-dicarboxylic acid in the above-mentioned publication.
3,4-Dimethoxythiophene has also been obtained by decarboxylation of 3,4-dimethoxythiophene-2,5-dicarboxylic acid (in the presence of Cu powder, at 180-190xc2x0 C.) without a solvent. C. Overberger, J. Am. Chem. Soc., 73 (1951), 2956-2957. The high temperature necessary for carrying out the reaction again stands in the way of industrial application. For application to EDT, the comments made in the previous section apply.
In the absence of diluents and metal catalyst, a yield of only 65% is achieved in the purely thermal decarboxylation of 3,4-dimethoxythio-phenedicarboxylic acid to dimethoxythiophene at 250xc2x0 C. See A. Merz, Chr. Rehm, J. prakt. Chem., 338 (1996), 672-674, in which a product mixture that is obtained must then be separated in a complex manner, i.e. in a number of steps.
EDT and similar 3,4-dialkoxythiophenes are very valuable materials for the preparation of conductive polymers. See, for example, G. Heywang, F. Jonas, Adv. Mater., 1992, 4, 116; F. Jonas, L. Schrade, Synthetic Metals, 41-43 (1991), 831-836.
A direct route to such thiophenes starts with the condensation of thiodiacetic acid esters with, for example, oxalic acid esters via 3,4-dihydroxythiophenedicarboxylic acid esters, which can be alkylated, saponified, and decarboxylated (see Hinsberg, Chem. Ber., 43, (1910), 904; and G. Heywang, F. Jonas, Advanced Materials, 4 (1992), 116).
A special form of this decarboxylation reaction has now been found.
The invention therefore relates to a process for the preparation of 3,4-dialkoxythiophenes of the general formula (I) or 3,4-alkylenedioxythiophenes of the general formula (II) 
comprising
(1) decarboxylating, respectively, a parent 3,4-dialkoxy-2,5-thiophenedicarboxylic acid of the general formula (Ill) 
xe2x80x83wherein R1 and R2 are straight-chain or branched alkyl having 1 to 15 carbon atoms, or
a parent 3,4-alkylenedioxy-2,5-thiophenedicarboxylic acid of the general formula (IV) 
xe2x80x83wherein X is optionally substituted xe2x80x94(CH2)nxe2x80x94and n is an integer from 1 to 12,
in a solvent or diluent that has a higher boiling point than the decarboxylated product and is not an aromatic amine, and
(2) separating the end product from the higher-boiling solvent or diluent by distillation.
The process according to the invention is particularly suitable for the preparation of 3,4-ethylenedioxythiophene (EDT, IUPAC name 2,3-dihydrothieno[3,4-b]-1,4-dioxin): 
and of the alkyl-substituted compounds derived therefrom, such as a compound of the formula 
or of 3,4-dimethoxythiophene.
The novel process allows the desired decarboxylation of dialkoxythiophenedicarboxylic acids to give dialkoxythiophenes to be carried out and the resultant products to be worked up in an elegant and simple procedure. The desired products are obtained in very good yield.
The invention is carried out by suspending the starting material, the dialkoxythiophenedicarboxylic acid, in a polar solvent or diluent that has a higher boiling point than the desired dialkoxythiophene. The solvent or diluent preferably has a boiling point which is at least 5xc2x0 C. higher. If the starting material is employed in aqueous solution or suspension, the water can be distilled off in situ in a first step by heating and distillation. A separate drying step, for example, in a drying cabinet or a paddle dryer, is thus unnecessary.
The decarboxylation is subsequently carried out at elevated temperature, and the product is finally distilled off from the solvent or diluent during or after the decarboxylation. The distillation conditions depend on the physical properties of the product and diluent and on the purity requirements. Thus, for example, a product mixture containing diluent can first be distilled off directly from the reactor and then subjected to rectification. However, the products can also be distilled off in a suitable apparatus, optionally using a column.
In general, a single distillation over a separating column is sufficient to obtain particularly pure products.
The diluent serves, inter alia, for dissipating and distributing the heat supplied via the reactor wall, thus avoiding local overheating.
The decarboxylation reaction can be carried out without a catalyst, i.e. at a temperature of 170 to 260xc2x0 C. In a preferred embodiment, decarboxylation is carried out in the presence of a catalyst, in which case significantly lower temperatures are sufficient, for example, in the range 100 to 180xc2x0 C. (preferably at temperatures of from 120 to 170xc2x0 C., particularly preferably from 130 to 160xc2x0 C.). The catalyst can be a heavy-metal compound, for example, a copper salt.
The solvents and diluents according to the invention can be, for example, silicone oils, ketones, esters, ethers, sulfoxides, sulfones, or alcohols. However, nitrogen bases, such as quinoline, are unsuitable since, even in traces, they impair the quality of the end products.
Particularly suitable are, for example, Baysilone(copyright) (commercial products from Bayer AG), polyethylene glycols, phthalic acid esters, diary ethers, tetramethylene sulfone, diaryl sulfones, and diaryl sulfoxides. Very particularly suitable are Baysilone(copyright) and polyethylene glycol 300 and 400, dibutyl phthalate, ditolyl ether, diphenyl sulfone, diphenyl sulfoxide, and tetramethylene sulfone.
For a suitable procedurexe2x80x94depending on the purity of the starting material employedxe2x80x94fresh starting material can be added to the reactor residue, which contains the majority of the diluent, and a new reaction cycle carried out. After a number of cycles, the diluent is then separated off from the dark byproducts, for example, by distillation, by addition of water, or in any other suitable manner, and can be recovered to a considerable extent and reused, which considerably improves the economic efficiency of the process.
In particular, the presence of a diluent simplifies the removal of secondary components, which in turn simplifies cleaning of the reactor after the production campaign.
The catalytically active heavy-metal compound that allows decarboxylation at lower temperature is, for example, basic copper carbonate, copper sulfate, copper oxide, or copper hydroxide.
In a particularly suitable embodiment, moist dialkoxythiophenedicarboxylic acid or alkylenedioxythiophenedicarboxylic acid is introduced into the diluent, heated to above the boiling point of water, and dried by removing water by distillation. If desired, heavy-metal catalyst is then added, the decarboxylation is carried out by heating to the requisite temperature, and the desired product is then removed by distillation (if desired, under reduced pressure). This distillation can initially be carried out without separation, but the distillation can also be carried out using a column at this stage, so that the dialkoxythiophene or alkylenedioxythiophene is obtained in the desired purity. From particularly high-boiling diluents, it is even possible to carry out the distillation at this stage without a separation stage.
Further purification methods for crude distillates that may need to be used if desired are known to the person skilled in the art. Particular mention should be made of washing and chromatography.
The procedure selected depends on external factors, such as, for example, the boiling behavior of the diluent compared with the product, the apparatus available and the cycle times desired.
The outlined variants are intended to describe the invention, but it is in no way limited thereby.